minor #14648 [Console] Update lockable_trait.rst (noniagriconomie)

This PR was merged into the 4.4 branch.

Discussion
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[Console] Update lockable_trait.rst

While attending to the `SymfonyWorld Online 2020 - Lock & Semaphore: The gatekeepers of your resources`
I remembered this feature and found this missed part
For upper branches v5.1+, the `Command::SUCCESS` is to be used

Commits
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a874705 Update lockable_trait.rst

Author: OskarStark

console/lockable_trait.rst

@@ -38,6 +38,8 @@ that adds two convenient methods to lock and release commands::
             // if not released explicitly, Symfony releases the lock
             // automatically when the execution of the command ends
             $this->release();
+            
+            return 0;
         }
     }